1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, as well as in “Thermally Processed Silver Systems” by D. H. Klosterboer, appearing in “Imaging Processes and Materials”, Neblette, 8th edition, edited by J. Sturge, V. Warlworth, and A. Shepp, Chapter 9, pages 279 to 291, 1989. All of the patents, patent publications, and non-patent literature cited in the specification are hereby expressly incorporated by reference herein. In particular, photothermographic materials generally have an image forming layer including a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images, dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed on the exposed region.
Methods of manufacturing such a photothermographic material using an organic silver salt include a method of manufacturing by a solvent coating, and a method of coating an aqueous coating solution using an aqueous dispersion of fine polymer particles or an aqueous solution of a water-soluble polymer as a main binder followed by drying. Since the latter method does not require a process of solvent recovery or the like, a production facility therefore is simple and the method is advantageous for mass production.
In the case of the photothermographic material having an aqueous-based coated image forming layer utilizing organic silver salts described above, the use of hydrophobic polymer latex as the main binder for the image forming layer to avoid adverse influence by moisture on photographic properties is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 10-10670, and the additional improvement thereof leads to formation of a clear image. However, under various utilization conditions, such as storing the photothermographic material at high temperature and humidity, the surfaces of the photothermographic material are liable to be adhered to each other, whereby separation thereof often causes defects such as the image forming layer being scratched or peeled off, and therefore improvement is needed.
Problems such as the defects described above are easily generated especially when a hydrophilic polymer derived from animal protein (for example, gelatin) is used in the outermost layer. A method for improvement is disclosed in JP-A No. 2002-162712, but the improvement does not reach a sufficient level and has a negative impact on lowering Dmax and deterioration in brittleness of the film. There is therefore a need in the art for improved photothermographic materials which do not exhibit the above negative effects.
In a process of thermal development, a photothermographic material is brought to contact with a heating means at a thermal developing section at a back surface or a surface having an image forming layer of the photothermographic material. As the heating means, either a drum type heater or a plate type heater can be used. A preferable process of thermal development by a plate type heater is described in JP-A No. 11-133572, which discloses a thermal developing apparatus in which a visible image is obtained by bringing an imagewise exposed photothermographic material into contact with a heating means at a thermal developing section, wherein the heating means comprises a plate heater, and a plurality of pressing rollers which are oppositely provided along one surface of the plate heater.
The back surface which contacts with the heating means is modified to contain a lubricant and a fluorine-containing surface wetting agent in a layer coated at the back surface for lowering friction resistance of the back surface and transporting smoothly the photothermographic material.
However, these materials added in the back layer are transferred in part during the thermal developing to the heating means, and accumulated on a surface of the heating means, and therefore it results in an unevenness of a developed image or an occurrence of jamming trouble by slipping due to insufficient transportability.
Therefore, a means to improve the transportability which can transport smoothly the photothermographic material during the thermal developing without these troubles is expected.